The present invention relates generally to resonant frequency tags and, more particularly, to a method for making such resonant frequency tags to provide for improved control of the resonant frequency of such tags.
Resonant frequency tags are tags which include a passive electrically resonant frequency circuit which resonates at a predetermined frequency when stimulated by a radio frequency electromagnetic field at about the resonant frequency of the tag and which is incident upon the tag. A resonant frequency circuit resonating within a region occupied by such an electromagnetic field perturbs the electromagnetic field. The perturbation of the electromagnetic field is detectable by suitable equipment. Consequently, the presence of a passive resonant frequency tag within a prescribed region may be detected.
Typically, resonant frequency tags are attached to or embedded within goods sold at retail or to the packaging for such goods in order to deter or detect theft. Resonant frequency tags used for this purpose are capable of being removed from the goods or deactivated when a legitimate sale is consummated. Resonant frequency tags which are not removed or deactivated at the point of sale may be detected by suitable detection apparatus generally placed at points of exit from a retail or other establishment. Such resonant frequency tags may have other uses including for identification or information purposes, such as a radio frequency identification (RFID) tag which may or may not include an integrated circuit or chip.
Typically, a resonant frequency tag comprises a generally flat thin laminate of a dielectric layer separating two conductor layers. Typically, one of the conductor layers comprises a flat spiral conductor (coil) forming an inductor, and a land forming one plate of a capacitor which is connected to a proximal end of the coil. A second land forming a second plate of the capacitor is formed as the second conductor layer. A through connection between the second plate and a distal end of the coil completes the resonant frequency circuit comprising the coil inductor connected parallel with the capacitor.
It is required that the inductive and capacitive elements of resonant frequency tags be manufactured with some precision in order that the resonant frequency of the tags be held within prescribed limits of the detection apparatus. A generally used method for making resonant tags employs etching a metallic foil to form the components of the conductive layers.
The manufacturing techniques employed in producing the prior art and current resonant frequency tags results in some unwanted variability in the final tag frequency. The unwanted variability is generally the result of small changes in the value of the capacitive element which vary from resonant circuit to resonant circuit during the production process. Such variations in the value of the capacitive element may be due to several factors, including irregularities in the dielectric area between the plates of the capacitor. The present invention comprises a method for compensating for variations in the manufacturing process to produce resonant frequency tags with a more consistent resonant frequency.